Some control systems include valves that are motor driven. In many of these systems, the motor driver circuitry includes a relatively large capacitor bank to provide local power to the motor driver. For efficient operation, the capacitor bank is charged to the same voltage as the supplied power rail. During initial power-up, or numerous other events during which the input voltage ramps up, a relatively heavy inrush current charges the capacitor bank.
As may be appreciated, the motor driver circuitry is preferably provided with protection circuitry to provide protection from an external short circuit. In particular, the protection circuitry is preferably configured to detect if the current increases beyond a predetermined limit, and immediately shut the motor driver circuitry down before any component failure.
In some aircraft, environmental interference, such as audio frequency interference, with the power lines could cause repeated inrush currents as the capacitor bank is being charged. This is potentially more troublesome than power-up inrush current. This is because the power-up inrush current occurs only once, whereas the environmental interference may cause the inrush to repeat multiple times in a short period. There are various other scenarios in which it is preferable to distinguish relatively heavy inrush currents a fault current. The challenge is to design a circuit that detects short circuit faults, and that is able to distinguish the heavy currents caused by capacitor charging from short circuit faults.
A traditional method for distinguishing these two events is to implement a time delay before detecting an over current. With this method, an overcurrent condition is detected only if the overcurrent condition exists for a certain time period. This method can be ineffective in many scenarios because the time to charge the capacitor bank is sufficient to damage the circuit in case of a short circuit. Also in the case of a short circuit, the power supply input will not see heavy current until the capacitor bank discharges through the short circuit, which can damage components. To overcome this shortcoming, the short circuit needs to be detected as early as possible, which does not allow for a delay to accommodate the current for charging the capacitor bank.
Another method is to clear an overcurrent latch once after providing sufficient time to charge the capacitors. Only in cases where the overcurrent occurs multiple times in a short span will an overcurrent condition be considered and the motor driver shutdown. This method will not work in scenarios like the above-described audio frequency interference, where an overcurrent may be detected multiple times.
Hence, there is a need for a system and method that is capable of distinguishing between a fault/short circuit overcurrent and an overcurrent that may be caused by capacitive load/capacitor bank charging/discharging. The present invention addresses at least this need.